In this series, we are looking at how it is that consciousness is worth its biological cost and in this post we come to what some would consider the essence of consciousness – global awareness. The content of consciousness is widely available in the brain. It seems that areas/processes of the brain have varying amounts of connectedness. The brain is more or less at the limit of the amount of white matter it can house to connect all its gray matter. If thought is restricted to neurons firing and connected by axons and dendrites, consciousness appear near impossible because there seems no way for every signal to pass to spatially and functionally distant areas.
I am dyslexic and long before there was modern theories about what this problem really was, I had the feeling of a blocked path and the need to find a different path around the block. In the days of my youth there was no diagnosis or therapy for dyslexia so I (later with my husband) had to develop ideas of what was wrong and therapies to deal with the problem. One attempt was to learn to read Russian script out loud and write it from dictation, without learning the meaning of any words. The idea was to practice a form of phonetics while bypassed semantics. I studied etymology to try and separate spelling from sound. I did cryptic crosswords. Slowly connections were made and slowly they stopped needing consciousness to be effective. I still have dyslexia in that I ‘hear’ whole words and syllables rather than phonemes and this still causes some problems in reading, writing and learning a new language, but I have learned how to cope. I am convinced that not every part of the brain can connect directly with every other. If the foundation for connection is not there (as one is missing in dyslexia), it will not just appear because you need it.
When we learn new things (say playing the piano), we start by working at a conscious level and practicing until we no longer need to be aware of the find detail of what we are doing – they become automatic. We are learning new things all the time – new people, new places, new things, new tasks and so on. If consciousness was not there, we would only be able to learn those sorts of things that evolution had very specifically prepared us for.
But how is this global communication done? I have not encountered a clear authoritative description and so I assume that it is still a mystery to be solved. But there are things that seem likely. The availability of the content of consciousness has to do with rhythms (as do many biological functions).
If you are not familiar with biological rhythms, you may want to consider as an example: the heart muscle – it beats. If the heart cells are separated, the individual cells beat as beating is part of the physiology of being a heart muscle cell. But the beating is also contagious and a cell will contract if a neighbour cell contracts. On top of this there is a period of time after a contraction when the cell cannot contract again until it has recovered. In the intact heart the fastest beating cell captures the others. When it contracts, others do by a contagious wave, then all the cells enter an inactive state and the first one to beat next is that fastest one. They all beat to the fastest beat – that of the pace-maker cell. It is not just muscle cells throughout the body who beat, neurons can also maintain rhythms and their beating can be captured by a local pace-maker neurons. Hearts are simpler but the brain situation is much more complicated.
Without going into great detail, EEG activity has conventionally been described in terms of a set of wide frequency bands, usually defined from slowest to quickest as: delta (1.5–3.5 Hz), theta (3.5–7.5 Hz), alpha (7.5–12.5 Hz), beta (12.5–25 Hz), and gamma (25–50 Hz). Different functional systems produce these rhythms. The spectrum comes from complex homeostatic systems involving brainstem, thalamus, and cortex and utilizing all neurotransmitters. Pacemakers in the thalamus interacting with the cortex and other areas causes synchronous oscillation in the alpha range as sensory signals are feed into the cortex. The nucleus reticularis can hyperpolarize the thalamic neurons and slow this rhythm to the theta range, decreasing sensory input to the cortex. Theta activity is generated in the limbic system and associated with memory activity. Delta activity is believed to originate in neurons in deep cortical layers and in the thalamus. It is normally inhibited by input from the lower brain controlling the amount of alertness/sleep. Activity in the beta band reflects cortico-cortical and thalamo-cortical transactions related to specific information processing. Gamma activity reflects cortico-thalamo-cortical and cortical-cortical reverberatory circuits, which play an important role in perception. These frequencies are indications of synchrony in the firing of neurons and also of phase-locking of a faster rhythm to a particular place on a slower rhythm.
As well as pace-maker neurons firing, synchrony is also affected by local field potentials. Neurons fire when their membranes pass a threshold as the voltage difference between inside and outside decreases. So the outside environment is part of the threshold. If the local field potential becomes nearer to a neuron’s inside field, the neuron is more likely to fire. LFPs are the product of many influences including other neurons near by, the glial cells packed around the neurons and the glial calcium ion communication (also rhythmic). There are magnetic fields that may affect the activity of neurons. It seems to me that there are long years of work ahead in clarifying the causes. interactions and effects of brain rhythms. But it does seem that the contents of consciousness are ‘broadcast’ across the brain during a very particular wave of sustained synchronous activity from the frontal cortex back across the length of the cortex to the primary sensory areas involving the parietal and other parts of the cortex and the thalamus.
To illustration the sort of activity involved, here is a very interesting video. (here). The time is not speeded up or slowed down, the colors indicate frequency ranges, the sounds are not simplified but the frequencies only made auditory and moved into a good range for human hearing. The resolution of time and location is only as good as the original fMRI scan. One can almost see the communication of information involved in consciousness in this video.
There is more to come. Previous posts in this series: